Downhole seismic investigation consists in studying the structure of the underground geological strata on the basis of seismic signals received at various different levels in a borehole in response to a series of excitations generated by a distant seismic source. The source may be situated on the solid surface of the Earth, or else in an adjacent borehole.
The quality of the results provided by such an investigation depends to a very large extent on the quality of the seismic signals as recorded, and consequently on the quality of the coupling between the detector means and the ground, i.e. the wall of the borehole.
Investigations have shown that the body of the sonde carrying the detector means has modes of vibration which are excited during seismic emission. If the seismic detector means are rigidly connected to the body, then the vibrations to which the sonde body is subject may be communicated to the detector means, thereby affecting the quality of the recorded signals.
Patent documents FR-A-2,561,394 and FR-A-2,554,600 suggest using a resilient connection between the detector means and the sonde body. Document FR-A-2,561,394 describes a sonde having an anchor arm for pressing the sonde body against the wall of the borehole. On the side opposite to the anchor arm, the sonde body has a lateral cavity with the detector module including the seismic sensor being disposed therein. The module is connected to the sonde body by a resilient connection so as to provide a high degree of de-coupling between the module and the sonde. A hydraulic actuator serves to press the module against the wall of the borehole in order to perform seismic measurements, and to retract it into the cavity while the sonde is being moved along the borehole.
The drawback of the above devices lies in the fact that it requires hydraulic actuator members which are complicated and fragile; further, these members are bulky and heavy, thereby increasing the diameter and the weight of the sonde, which is disadvantageous.